Refactor ProseMirror example and Tree codes

[hackerwins]

What this PR does / why we need it?

Refactor ProseMirror example and Tree codes

• Refactor ProseMirror example

This commit improves the ProseMirror example by removing redundant and
unused code in both HTML and CSS, resulting in cleaner and simpler.

• Eliminate Tree.Split
  Since Tree.Split is invoked during content editing, this commit removes
  the separate Tree.Split, streamlining the codebase.

• Enhance Tree tests and uncomment basic text split test
  In preparation for adding a split parameter to Tree.Edit, this commit
  simplifies the Tree.Edit interface for better representation in a line.

Any background context you want to provide?

What are the relevant tickets?

Address yorkie-team/yorkie#662

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (859c019) 67.94% compared to head (81a4a7a) 68.03%.

❗ Current head 81a4a7a differs from pull request most recent head 4251c59. Consider uploading reports for the commit 4251c59 to get more accurate results

Files	Patch %	Lines
src/document/crdt/tree.ts	92.85%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   67.94%   68.03%   +0.08%     
==========================================
  Files          58       58              
  Lines        8783     8773      -10     
  Branches      795      794       -1     
==========================================
+ Hits         5968     5969       +1     
+ Misses       2555     2546       -9     
+ Partials      260      258       -2     

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